1. Field of the Invention
This invention relates to an electrophotographic method for color image formation. More particularly, it is concerned with an electrophotographic method capable of readily controlling tone of an entire color image, or each tone of a color image by appropriate regulation of an electrode voltage.
2. Description of the Prior Art
Heretofore, it has been a practice for the color image formation using an electrophotographic photosensitive member to expose a color image original for at least three times, the exposure being performed through color filters (generally, red, green and blue filters), and to form a toner image at every three exposures using a toner which is complementary to the color of each filter in accordance with the electrophotographic method. For instance, the photosensitive member is first charged, then an image exposure is done through a red filter, thereafter the exposed image is developed with cyan toner, and the thus developed image is transferred onto an image transfer paper. Subsequently, the exactly same process steps as mentioned above are followed with the red color filter being replaced by a blue or green filter, and the developing agent by magenta or yellow toner for the color image formation. In the case of adding an image forming process using black toner to more improve quality of the color image, there is further added a process step wherein the exposure is effected with a slightly stronger exposure light without use of a color filter to electrostatically adhere the black toner on the surface of the photosensitive member corresponding to the black portion in the original image.
In such color image forming method by the conventional electrophotographic process, it is necessary that the image exposure be done at least three times by changing the color filter with the consequence that the process steps become complicated. Furthermore, in the conventional method, corona charging process is effected during an interval between each of these three times exposures and developments. In this corona charging, however, control of the charge quantity is generally extremely difficult. Accordingly, the tone of the color image is controlled by adjusting the exposure quantity. It is again not so easy generally to control with high precision the color tone of the image by adjustment of the exposure quantity, and still more to independently control each tone of the color image.
In the formation of a polychromatic image including black by the conventional electrophotographic process, there has been adopted a method, wherein a black image is first formed with a black toner and then a color image and a black image are formed with color toners. This image forming sequence can, of course, be the reverse of the abovementioned. In this case, however, there are formed two kinds of black images, the one with the black toner and the other with the color toners exclusive of the black toner, so that, if registration of the images is not precisely done during the image exposure and image transfer steps in these two image formations, there would occur such a possibility that the color toner image identical with the black toner image is present near the black toner image in its unregistered state to render the reproduced image unsightly. In particular, in the case of an office copy of a document, the principal image such as letters, numerals, etc. is mostly in black, and there appear colored images only partially.
Formation of the black image alone from a polychromatic image including black could be done easily with the conventional method (i.e., by exposing the image with light of the same color as the coloring matter, etc.), but it is impossible to form the colored image alone with such conventional method.